In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to a power smoothing system of wind power generation or a midnight power storage system, a household dispersive power storage system based on solar power generation technology, a power storage system for electric vehicles, etc.
The number one requirement for cells used in these power storage systems is high energy density. Development of a lithium ion battery is advancing at a rapid pace, as an effective strategy for cells with high energy density that can meet such requirement.
The second requirement is high output characteristic. A high power discharging characteristics in the power storage system is required for the power storage system during acceleration, for example, in a combination of a highly efficient engine and the power storage system (for example, a hybrid electric vehicle), or a combination of a fuel cell and the power storage system (for example, a fuel cell electric vehicle).
An electric double layer capacitor, a nickel-hydrogen battery, etc. are currently under development as a high power storage device.
Electric double layer capacitors that employ activated carbon in an electrode has the output characteristics of about 0.5 to 1 kW/L. This electric double layer capacitor has been considered to be the most suitable device for fields requiring the high power, since it has not only high power characteristics but also high durability (cycle characteristics and high temperature storage characteristics). However, energy density thereof is only about 1 to 5 Wh/L. Accordingly, even higher energy density is necessary.
On the other hand, the nickel-hydrogen battery, which has been employed in current hybrid electric vehicles, has high power equivalent to that of the electric double layer capacitor, and has an energy density of about 160 Wh/L. However, research is being actively pursued toward further increasing their energy density and output characteristics, and increasing their durability (particularly, stability at high temperature).
Research is also advancing toward increased output in the lithium ion battery as well. For example, such a lithium ion battery are being developed that is capable of providing a high power of over 3 kW/L at 50% depth of discharge (value showing a state of what % of discharge capacitance of the storage element was discharged). However, it is designed to dare to suppress high energy density, which is the largest characteristics of the lithium ion battery, because energy density thereof is equal to or lower than 100 Wh/L. Durability (cycle characteristics and high temperature storage characteristics) thereof is inferior as compared with the electric double layer capacitor. Accordingly, such a lithium ion battery is used only in a narrower range than a depth of discharge range of 0 to 100%, to hold practical durability. Research is advancing at a rapid pace toward further increased durability, because practically usable capacitance is considered to be further decreased.
There is strong demand for implementation of the storage element exhibiting high energy density, high power characteristics and durability. However, each of these existing storage elements has its advantages and disadvantages. Accordingly, a new storage element satisfying these technological requirements has been required. The storage element known as a lithium ion capacitor are getting a lot of attention and being actively developed as a promising candidate.
The lithium ion capacitor is a storage element using a nonaqueous electrolytic solution containing a lithium salt (hereafter it may also be referred to as “nonaqueous lithium-type storage element”). It is a storage element which carries out charging and discharging by: non-Faraday reaction based on adsorption/desorption of anions, similar to an electric double layer capacitor, at about 3 V or higher, at the positive electrode; and Faraday reaction based on occlusion/releasing of lithium ions, similar to a lithium ion battery, at the negative electrode.
As for the electrode materials used in the storage elements, and characteristics thereof, in general, in the case where an activated carbon, etc., is used as a material for an electrode, and charging and discharging are carried out by adsorption/desorption (non-Faraday reaction) of ions at the surface of activated carbon, a high power as well as high durability are achieved, but an energy density decreases (for example, assuming it is 1). On the other hand, in the case where an oxide or a carbon material is used as a material for the electrode, and charging and discharging are carried out by Faraday reaction, an energy density is increased (for example, assuming it is ten times larger than that is achieved by non-Faraday reaction, using activated carbon), but there is a problem in their durability and output characteristics.
The electric double layer capacitor is characterized in that, among the above electrode materials, an activated carbon (having 1 energy density) is used in a positive electrode and a negative electrode, and charging and discharging are carried out by non-Faraday reaction at both of the positive and negative electrodes, and therefore it has low energy density (1 at the positive electrode×1 at the negative electrode=1), although it has a high power, as well as high durability.
The lithium ion secondary battery is characterized in that a lithium transition metal oxide (having 10 energy density) is used in a positive electrode, and a carbon material (having 10 energy density) is used in a negative electrode, and charging and discharging are carried out by Faraday reaction at both the positive and negative electrodes, and therefore it has high energy density (10 at the positive electrode×10 at the negative electrode=100), but it has a problem in their output characteristics and durability. Furthermore, a depth of discharge is limited in order to achieve a high durability, which is required for hybrid electric vehicles, and energy can be used in a lithium ion secondary battery is only 10 to 50% thereof.
The lithium ion capacitor is a asymmetric capacitor characterized in that an activated carbon (having one energy density) is used in a positive electrode, and a carbon material (having ten energy density) is used in a negative electrode, and charging and discharging are carried out by non-Faraday reaction at the positive electrode, and by Faraday reaction at the negative electrode, and therefore it has characteristics of both the electric double layer capacitor and the lithium ion capacitor. Further, the lithium ion capacitor is characterized in having a high energy density (1 at the positive electrode×10 at the negative electrode=10), although it has a high power and high durability, and different from the lithium ion secondary battery, a depth of discharge is not necessarily limited.
PATENT LITERATURE (PTL) 1 proposes a lithium ion secondary battery using a positive electrode comprising lithium carbonate in a positive electrode, and having a current shutdown mechanism that works in response to increase of an inner pressure of the battery. PTL 2 proposes a lithium ion secondary battery using a lithium composite oxide, such as lithium manganate, etc., as a positive electrode, wherein an elution of manganese is reduced by adding lithium carbonate in a positive electrode. PTL 3 proposes a method for recovering a capacitance of a deteriorated storage element, by oxidation of various kinds of lithium compounds, as materials to be oxidized, at a positive electrode. PTL 4 proposes a method for preventing a capacitance from decrease during charging and discharging cycles, and increasing an initial capacitance, by adding lithium carbonate to a composite oxide containing lithium and nickel in a positive electrode.